My opinion is that belief can help or hurt you. You can do a spell without believing it will work, but it can work anyway. But if you have belief, it
Is most likely stemmed from a want or need and interest, such as you wanting power and you have an interest in fantasy. You find a whole website and religion dedicated to magick that you'd find in fantasy, and it could gain you power, you're most likely to take more action with the spell and you'd be more motivated to your dedication or study. But it can hurt you as well.
You may want to use magick to manipulate such natures that don't exist within the magick pantheon, such as hurling fireballs at a whole city to take it over and enslave its inhabitants. That clearly cannot be done...so if someone were to cast a spell to turn them into a powerful giant wizard, you'd find it won't work or the results aren't as anticipated...it only hurts you because it's unpredicted results that could harm you and discourage you as well, swaying you away from the true natures of magick each time a spell doesn't work. At the end of the day, it comes to balance and reality. You have to know your limits.